Physics- Unit 1 - Energy Transfer by heating

Infra-Red radiation

Infra- Red Radiation

Infra-red raditation is heat transfer by electromagnetic waves. All objects emit Infra-red radiation. The hotter an object is, the more infra-red radiation it will emit.

In this thermal image there is an elephant. Its back is red (hot) because the back absorbed most of the heat during the day, the arch of the back is about 97.5oF whilst the rest of the body is yellow meaning it is cooler.

A Leslie Cube has different coloured sides. The dull black side is the best emitter of Infra- Red Radiation and the shiny surface is the worst. The best absorber of thermal radiation is the matt black side and the worst is the shiny surfaces.

Black surfaces are the best emitters of heat radiation.Black surfaces are the best absorbers of heat radiation.Shiny surfaces are the best reflectors of heat radiation.Shiny surfaces are poor absorbers of heat radiation.Glass will not transmit heat radiation.

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Kinetic Theory

Solids

Particles very close and held together by bonds, can only vibrate.

Liquids

Particles close together but can move around.

Gases

Particles very far apart and moving very quickly.

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Definitions

Variables

Catorgoric- These tell us the name of the variable, e.g the type of wire used for a resistance investigation.

Continuous- A continuous variable can be any numerical value, e.g the length of wire used in a resistance investigation.

Dependent- The variable that you are measuring as a result of changing the independent variable, e.g current measured in a resistance investigation.

Independent- The variable that you have decided to change in an investigation, e.g the length of wire used in a resistance investigation.

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Definitions

Errors

Human- Simply an error made by a human. This is usually used to differentiate the source of errors from other causes such as measurement, random, variation, machine failiure, lack of calibration and improper design.

Random- Measurements when repeated are rarely exactly the same. If they differ randomly then it is probably due to human error when carrying out the experiment.

Systematic- If the data is inaccurate in a constant way, e.g all results are 10mm more than they should be, this is often due to the method being routinely wrong.

Zero- A systemic error, often due to the measuring instrument having an incorrect zero.

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Conduction

Metals are good conductors because they have Free Electrons. Heat is conducted from regions of high temperatures to regions of low temperatures. Wood, which is an insulator, is a poor conductor because it does not have Free Electrons.

The uses of convection

When a gas or liquids is heated its volume increases and its density decreases. The gas and liquid will rise by convection.

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Evaporation

Evaporation is a type of vapourisation of a liquid that occurs only on the surface of a liquid.The other type of vaporisation is boiling, which instead occurs within the entire massof the liquid.Evaporation is an essential part of the water cycle. The sun (solar energy) drives evaporationof water from oceans, lakes, moisture in the soil and other sources of water.Evaporation of water occurs when the surface of the liquid is exposed, allowing molecules to escape and form water vapour; this vapour can then rise up and form clouds.At its boiling point, of 100ºC, water will rapidly be turned into vapour, as theenergy supplied to the water is enough to break all the molecular bonds in water apart.At temperatures between 100ºC and 0ºC, however only some of the molecules in the waterhave enough energy to escape to the atmosphere.

The particles in a liquid have different energies. Some will have enough energyto escape from the liquid and become a gas. The remaining particles in the liquid havea lower average kinetic energy than before, so the liquid cools down as evaporationhappens.

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Evaporation

The rate of evaporation will depend upon a number of factors. Rates increase whentemperatures are higher; an increase of 10ºC will approximately double the rate ofevaporation.

The humidity of the surrounding air will also influence evaporation. If the air isextremely humid there will already be a lot of water vapour in the air, not allowingas much of the water to evaporate. So the presence of wind will also increaseevaporation. On still days, water evaporating into the air remains close to its source,increasing the local humidity. As the moisture content of the air increases,evaporation will decrease. If however, a steady flow of air exists to remove the newlyformed vapour, the air surrounding the water source will remain dry.Another factor affecting the rate of evaporation is the amount of surface area. Thegreater the surface area exposed to the surroundings the faster it will evaporate.Temperature of the surrounding area also affects evaporation. If you put somewater in a cold environment and some water in a warm environment the warm waterwill evaporate faster as there is more energy in warm air.A final factor is wind speed. The more wind blowing across the surface of theliquid means there will be a constant flow of air with no water vapour in it rightabove the liquid, resulting in a faster evaporation.

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Condensation

Condensation is the change of the physical state of matter from gas to liquid and is the reverse of evaporation.Condensation is the process where water vapour in the atmosphere is returned to itsoriginal liquid state. In the atmosphere condensation may appear as clouds, fog, mist,dew or frost, depending on the physical conditions of the atmosphere.

Condensation is not a matter of one particular temperature but of adifference between two. Condensation of water vapour occurs when thetemperature of air is lowered to its dew point.The particles in a gas have different energies. Some may not have enough energyto remain as separate particles, particularly if the gas is cooled down.They come close together and bonds form between them, energy is released whenthis happens. All air contains water vapour of varying quantities.

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Condensation

The lower the air temperature, the smaller the maximum possible capacity for vapour.
When air is cooled, relative humidity increases, until at a particular temperature,
called the dew point, the air becomes saturated. Further cooling below the dew point
will induce condensation of the excess water vapour.

The temperature of the dew point will depend upon the absolute content of water vapour, that is the absolute humidity, measured in g/m3 (grams per metre cubed). The dew point of humid air will be higher than the dew point of dry air. Both air temperature and absolute humidity will determine what type of condensation will occur when the air is cooled. If air in contact with the ground is cooled to its dew point, dew or frost will form, dew if the point is above 0ºC, or frost if it is below 0ºC. Cooling of a larger layer of air near to the ground may produce mist or fog, which freezes if the dew point is below 0ºC. Air that is cooled to its dew point by rising and expansion will condense to form clouds.Above 0ºC, small droplets of water are formed. Condensation may also result in ice crystals at temperatures well below 0ºC. When temperatures are near or a little below 0ºC, supercooled water droplets can form.

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Condensation

When water turns to a liquid there is a considerable lowering of the volume of theatmosphere, due to the vapour turning to a liquid. The water particles are being pushedthrough a distance to accumulate as the liquid. The surface on which the condensationtakes place needs to be relatively cold to create a situation where the water comes out ofsolution in the air.When water condenses it gives out a lot of heat, this risks warming the surface so thewater will stop or slow it’s condensation. The rate at which this heat may be removed istherefore strongly connected to the rate of condensation. Another factor is the rate atwhich water vapour may reach the surface. This could depend on factors such astemperature and pressure and the amount of water dissolved in the air.One further factor that affects the rate of condensation is that the higher the humidity,the more water vapour there is in the air. Secondly the lower the temperature of the air,the lower the amount of humidity it can hold. This humidity is usually measured as apercentage, 100% means that the air cannot hold any more water vapour.As the air cools down it causes more and more vapour to be turned into water.

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Condensation

Condensation is when a gas turns into a liquid. The amount of condensation on a surface can be increased by reducing the temperature of the surface and increasing the surface area of the surface.

Energy TransferThe amount of energy loss from an object is affected by:-
Shape, size & type of material of object, Temperature difference and Material object is in contact with

Insulating Buildings

Loft Insulation- such as fibre glass reduces the rate of energy transfer through the roof. Fibre glass is a good insulator. The air between the fibres also helps to reduce the rate of energy transfer by conduction.

Cavity Wall Insulation- reduces heat loss through the walls. We place insulation between the two layers of brick that make up the walls of a house.

Double Glazed Windows- has two panes with dry air or a vacuum between the panes. Dry air is a good insulator so it cuts down heat conduction. A vacuum cuts out heat transfer by convection as well.

Aluminium Foil- between a radiator panel and the wall reflects heat radiation away from the wall.